# 2012/11/10 (SL)
gap> l :=  [ 2, 3, 4, 1, 5, 10, 9, 7, 8, 6, 11 ];;
gap> SortBy(l,AdditiveInverseSameMutability);
gap> l;
[ 11, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 ]
